Lore

Deep within Drangleic Castle, the Throne of Want is the "true throne", and taking it means to link the fire of the First Flame.

When the door to the throne is opened, the one who ventures inside will have to face these foes:

Throne Watcher and Throne Defender: This mysterious duo watched and protects the Throne of Want. They have been here for ages, watching and waiting for the next Monarch. But their souls have an odd taint…

Nashandra: The Queen, Nashandra, has a secret, Dark and ancient. She convinced Vendrick to cross the sea and pilfer a mighty prize from the Giants. She brought a Dark-like peace to the Kingdom of Drangleic, but eventually Vendrick realized something was amiss. He fled and refused to take part in Nashandra's plans any longer. But now she has found you, the new heir to the Throne of Want…

Theories

The Giant's Kinship and the Throne

The Throne of Want door opens to Giant's Kinship because that's how Vendrick designed it - he wanted to Link the Fire back to the Giants, so they would be cursed with the endless cycle rather than humans. That was the threat Nashandra had told of Vendrick, and that's how they could ultimately end the cycle, and Vendrick had stolen a Giant's essence to link the Fire back to them. Sensing their own downfall, a Giant hero eventually rose up - their parallel of the Chosen Undead - and lead the assault on Drangleic, to try and retrieve their essence so Vendrick couldn't link the curse back to them. That's when Vendrick found out Nashandra wanted an entity with a large enough soul that she could consume, to ensure the Fire would be extinguished once and for all.

Sources

Aldia Dialogue
Seek the throne.
Seek light, Dark and what lies beyond…
…
The throne will certainly receive you.
But the question remains…
What do you want, truly?
Light? Dark? Or something else entirely…

King's Ring
A soul is much like a curse.
And those with strong souls must bear
within themselves an even stronger curse.
Vendrick, King of Drangleic, used a
powerful soul to keep the curse at bay.
In the end, he sought the flame that began it all.
Wear this ring to open the King's Gate.

Giant's Kinship
Each king has his rightful throne.
And when he sits upon it,
he sees what he chooses to see.
Or perhaps, it is the throne,
which shows the king only what he wants.
The flames roar, but will soon begin to fade,
and only a worthy heir might burnish their light.
What is it, truly, a claimant of the throne
could desire?

Throne Defender Soul
Soul of the Throne Defender.
The throne of the King is sacred,
and requires an appropriate guardian.
Use the special soul of the Defender
to acquire numerous souls,
or to create something of great worth.

Throne Defender Set
Worn by the Throne Defender.
The Defender has stood by the throne for ages.
Will his wait be worth the while?

Throne Watcher Soul
Soul of the Throne Watcher.
The throne of the King is sacred,
and must be observed closely at all times.
Use the special soul of the Watcher
to acquire numerous souls,
or to create something of great worth.

Throne Watcher Set
Worn by the Throne Watcher.
Has high magic defense.
The Watcher waited by the throne for ages.
Will all his waiting ever be rewarded?

Soul of Nashandra
Soul of Nashandra, Queen of Drangleic.
The fragments of the abyss, of untold origin,
nourished their beings by the sides of would-be
monarchs. Perhaps they were simply ordeals
on the road to kinghood.
One day, the flames will fade, and only darkness
will remain. Unless, of course, an heir arrives.
Use the special soul of this prisoner of desire
to acquire numerous souls,
or to create something of great worth.

Bow of Want
A bow created from the soul of Nashandra.
The old one of the Abyss was reborn in death,
split into minuscule fragments, and spread
across the land.
Light and Dark are two sides of the same coin,
much like the soul and the curse. The beings
who presented themselves to those in search
of kinghood were drawn to their awesome strength.

Chime of Want
Sacred chime created from the soul of Nashandra.
The old one of the Abyss was reborn in death,
split into miniscule fragments, and spread
across the land.
After taking their shapes, they courted monarchs
of considerable power, which they desired
to make their own.

Scythe of Want
A Scythe born from the soul of Nashandra.
The old one of the Abyss was reborn in death,
split into minuscule fragments, and spread
across the land. After regaining their shapes,
they crawled forth, yearning for strong souls,
in search of greater power.